This invention relates generally to on-machine supercalendering of paper and, more particularly, to methods and apparatus for on-machine supercalendering of paper leaving from the output side of a paper machine and wherein both hard and soft calender rolls form soft calendering nips with each other.
The calendering of a paper web leaving the discharge end of a paper machine is a well known final finishing treatment for determining the smoothness and gloss of the surfaces of the paper as well as its consistency. Such calendering is generally accomplished by guiding a continuous paper web successively through a series of nips formed by calendering rolls.
Conventionally, a paper web is calendered in a so-called calender stack situated immediately adjacent to the output side of the paper machine and, when required, the treatment is accomplished in a so-called supercalender.
Calender apparatus comprise calendering rolls which define calendering nips through which the web is passed. Such calendering rolls constitute either "hard" rolls or "soft" rolls. It is understood that as used herein, the term hard rolls designates rolls formed, for example, of chill casting or steel, the hard surfaces of which have been ground smooth. The term soft rolls as used herein designates rolls whose surfaces are made of flexible material. For example, a flexible material generally used for such soft rolls is paper wrapped in layers around the shaft of the roll and compressed to form a uniform roll coating.
Furthermore, as used herein, the term "soft nip" designates the contact line between a soft roll and a hard roll. The term "hard nip" is used to designate the contact line formed between two hard rolls.
It is possible depending upon the type of paper and the requirements therefor to machine finish the paper web in single nip calender, i.e., a calender comprising only one pair of rolls. In most cases, however, a calender stack will comprise between four and eight rolls forming three to seven nips. In fact, in separate calender stacks for machine-finishing there can be as many as ten pairs of nips.
It is usually an object to machine finish paper so that both sides of the paper have an equal gloss. Accordingly, at least two soft nips are generally provided located in a manner such that both surfaces of the paper web are pressed against the surface of a hard roll.
In connection with improving the efficiency of paper production, it has proven important to provide a calender unit in which both the functions of a machine-finishing unit as well as a supercalender are combined. In this connection, applicant's assignee's Finnish patent application No. 761764, corresponding to U.S. Pat. No. 4,128,053, discloses an on-machine supercalender apparatus adapted to be incorporated in a paper machine. Such supercalender comprises a roll stack including conventional hard rolls and essentially the same number of soft rolls which are located outside of the roll stack to form soft nips against the hard rolls.
In such a calender unit which constitutes a combination of a machine-finishing stack and a supercalender, the paper web can be supercalendered as desired immediately after the same leaves the paper machine without any intermediate phases. However, the results obtained are not entirely satisfactory in that the so-called super gloss effected by the calendering treatment is not uniform. In other words, some areas of the surface of the paper are glossier than other areas. Furthermore, it has been found that the paper web subjected to the calendering treatment turns a blackish color resulting at least partially from the fact that the hard nips are in fact too hard and unduly inflexible.
Other problems have arisen in connection with calendering paper webs. For example, it has been found that soft rolls of a calender, such as paper rolls as described above, are easily damaged during operation. In conventional calenders which are composed of groups of three or more rolls, it has not been possible to replace a soft roll when the same is damaged without breaking the web. In certain conventional four nip calenders, although it is possible to change a soft roll during operation without having to break the web, such results in one surface of the web being untreated or uncalendered.
As to the state of the art, reference is also made to Finnish Patent No. 55694, corresponding to U.S. Pat. No. 4,131,063 and applicant's Finnish patent applications No. 793200 and 793201, corresponding to U.S. Pat. Nos. 4,332,191 and 4,326,456, respectively.